Methods for the production of cast iron with spheroidal or nodular graphite or graphite forms other than flake normally involve the treatment of cast iron in the liquid state with suitable nodularisers such as Mg, Ca, Na, Li, Sr, Ba, Ce, Di, La, Yt and compounds and alloys thereof.
Many processes have been devised to introduce these nodularisers to cast iron either by direct introduction into a pouring ladle or by use of separate more complicated equipment.
A widely employed method is to place the nodulariser on the bottom of the pouring ladle and then tap molten metal onto it.
In this method the nodulariser may also be covered with steel punchings or inert material.
Other known methods make use of gas agitation and some rely on plunging the nodulariser below the metal surface.
All of these processes suffer certain limitations in that in some instances reliability of treatment is poor and excessive fume, flare, pyrotechnics and metal splashing are experienced due to the violence of reaction between metal and nodulariser.
Another known method involves introducing the reactive additive into a stream of the metal to be treated. One such method is described in U.K. Patent Specification No. 1,076,456 as being particularly suitable for introducing an alloying material into a steel melt. The apparatus described in U.K. Specification No. 1,076,456 includes a treatment chamber into which an alloying material is introduced through one opening simultaneously with a stream of molten metal through another opening. A closure body provided with passages is provided at the base of the treatment chamber in order to provide for accumulation of the melt with the alloying material. The treated melt is then conveyed through the passages in the closure body into a collecting ladle. The function of the treatment chamber described in Specification No. 1,076,456 is not to hold additives but to control the metal stream so as to permit introduction of additives into the stream. The apparatus includes elaborate provisions for the separate introduction of the alloying material and for scavenging the apparatus prior to use. Moreover, fume and pyrotechnics associated with introduction of the alloying material into the metal stream are not avoided as indicated by the provision of fume extraction equipment.
Our prior European patent application No. 79302553.7 (Published No. 0011478) described and claimed a treatment unit which is adapted to be located in, or above a pouring ladle, the treatment unit comprising a pouring bush opening directly or indirectly into a container holding solid additive and which is provided with a cover and, in its base and/or peripheral walls, with a plurality of holes to allow the passage therethrough of molten metal; and treatment unit also comprising means for restricting contact of molten metal being treated with the atmosphere selected from a cover for the pouring ladle and a funnel adapted to accommodate the container. This apparatus solved some of the problems associated with previous proposals but we found that the number of treatments that was possible with one container was limited and the container life was relatively short due to failure of the refractory material.
We have, therefore, sought to minimise the problems of these various prior proposals.